Oil impregnated bearings are cheaper than ball bearings and fluid dynamic bearings and, therefore, are utilized for a number of applications as a substitute for these bearings in electric accessories for automobiles and various electric appliances. The oil impregnated bearings are generally produced through a compacting step, a sintering step, a sizing step, and a cleaning (degreasing) step. In the sizing step, a machining oil of a mineral oil has been hitherto used. The known machining oil, however, is unsatisfactory with respect to the machinability because of failure to form sufficient oil films. Further, because of poor degreasing efficiency, the mineral oil-type machining oil tends to remain in a large amount in the sintered metal. Further, since the remaining oil is poor in compatibility with the impregnated oil and sintered metal used in oil impregnated bearings, generation of sludge is caused.
In this circumstance, there is a demand for a lubricating oil for sizing which exhibits excellent machinability, degreasing efficiency, and compatibility with the impregnated oil and sintered metal used in oil impregnated bearings. With regard to published documents, Patent Document 1 discloses the use of an oil which is the same as a bearing oil. Patent Document 2 discloses a rapeseed oil. There is, however, a room left for the improvement in the known oils with respect to their performance.
[Patent Document 1] JP-A-H06-264110 (page 2)
[Patent Document 2] JP-A-H08-209370 (page 2)